Nuclear Resonance Fluorescence results when a nucleus is excited by photon absorption and then subsequently decays via photon emission to lower lying states of that nucleus. The decay is often but not always to the ground state. The emitted photon energy that results from a decay may be characteristic of the specific isotope which has decayed and therefore the detection of a photon of that energy may allow the identification of the presence of that isotope.
Because the emitted photon energies in NRF are in the MeV region, the photons involved may be very penetrating. This may allow NRF to be used for the non-intrusive inspection of dense cargo or materials. See U.S. Pat. No. 5,115,459, Explosives Detection Using Resonance Fluorescence of Bremsstrahlung Radiation, U.S. Pat. No. 5,420,905, Detection of Explosives and Other Materials Using Resonance Fluorescence, Resonance Absorption, and Other Electromagnetic Processes with Bremsstrahlung Radiation, and U.S. Pat. No. 7,120,226, Adaptive Scanning Of Materials Using Nuclear Resonance Fluorescence Imaging, the contents of all of which are hereby incorporated by reference.
The energies of the photons that are resonant with a specific isotope are for the most part determined by the nuclear structure of that isotope, and the nature of the strong nuclear interactions that bind that nucleus. Small effects may arise, however, from the recoil of the nuclear isotope due to the conservation of energy and momentum upon photon absorption and emission.
In particular, when a nuclear isotope absorbs a photon the energy of the absorbed resonant photon may not be simply the energy difference between the ground state of the nucleus and the resulting excited state of the nucleus. The photon energy must also account for the energy of excitation of the molecule or crystal to which the nucleus is bound. In a molecular structure, the molecule is generally excited because of the violent recoil of the nucleus caused by the conservation of momentum upon photon absorption. For light and heavy nuclei this recoil may be sufficient not just to excite the molecule but also to break the molecular bond. For crystalline materials, the recoil of the nuclear isotope may excite vibrations of the crystal in one or several of its many normal modes. The recoil may also break the nuclear isotope from its lattice position causing it to recoil almost freely through the crystal.